Wood fibre based direct pressed laminated flooring usually comprises a core of a 6-12 mm fibre board, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like material.
A laminate surface generally comprises two paper sheets, a 0.1 mm thick printed decorative paper and a transparent 0.05-0.1 mm thick overlay paper applied over the decorative paper and intended to protect the decorative paper from abrasion. The print on the decorative non-transparent paper is only some 0.01 mm thick. The transparent overlay, which is usually made of refined fibres (e.g. α-cellulose fibres), comprises wear resistant particles such as small hard and transparent aluminium oxide particles. The refined fibres are rather long, about 0.5-5 mm and this gives the overlay paper the required strength. In order to obtain the transparency, all natural resins that are present in the virgin wood fibres, have been removed and the aluminium oxide particles are applied as a very thin layer over the decorative paper. The surface layer of a laminate floor is characterized in that the decorative and wear properties generally are obtained with two separate layers one over the other.
The printed decorative paper and the overlay are impregnated with melamine formaldehyde resin and laminated to a wood fibre based core or particleboard core under heat and pressure.
The aluminium oxide particles may have a size in the range of 20-100 microns. The particles may be incorporated in the surface layer in several ways. For example they may be incorporated in the pulp during the manufacturing of the overlay paper. They may also be sprinkled on the wet lacquer during impregnation procedure of the overlay or incorporated in the lacquer used for impregnation of the overlay.
The wear layer may also be produced without a cellulose overlay. In such a case melamine formaldehyde resin and aluminium oxide particles are applied as a lacquered layer directly on the decorative paper with similar methods as described above. Such a wear layer is generally referred to as liquid overlay.
With this production method a very wear resistant surface can be obtained and this type of surface is mainly used in laminate floorings but it may also be used in furniture components and similar applications. High quality laminate floorings have a wear resistance of 4000-6000 revolutions, which corresponds to the abrasion classes AC4 and AC5 measured with a Taber Abraser according to ISO-standard.
It is also known that the wear resistance of a lacquered wood surface may be improved considerably by incorporating aluminium oxide particles in the transparent lacquer covering the wood surface.
The most common core material used in laminate floorings is fibreboard with high density and good stability usually named HDF—High Density Fibreboard. Sometimes also MDF—Medium Density Fibreboard—is used as core. Other core materials such as particleboard are also used.
Recently new “paper free” floor types have been developed where powder technology is used to obtain a solid laminate surface comprising a substantially homogenous mix of processed or non-processed wood particles, binders and wear resistant particles. Such floor types are below referred to as “solid laminate” floors. The wear resistant particles are preferably aluminium oxide particles and the binders are preferably thermosetting resins such as melamine formaldehyde resin. Other suitable materials are for example silica or silicon carbide. In general all these materials are preferably applied in dry form as a mixed powder on a HDF core and cured under heat and pressure to a 0.2-1.0 mm solid laminate layer. The solid layer in solid laminate floors provides high impact and wears resistance.
Powder technology is also used to produce floors with a surface that combines the paper technology and powder technology. Such floor types are below referred to as “wood powder laminate floors”. A decorative paper is applied on a sub layer comprising wood powder and binders. The decorative paper is protected by a conventional overlay. The main advantages with such floors are that deep embossing and improved impact resistance may be obtained.
Powder technology can also be used to produce floors with a powder overlay arranged on a decorative layer. The powder overlay comprises wood particles, a binder and wear resistant particles such as aluminium oxide. The powder overlay replaces a conventional overlay and provides improved wear resistant properties compared to the conventional overlay.
The standard production of powder mixes for manufacturing of decorative surfaces for application areas such as flooring panels, worktops, wall panels, furniture and others, are based on a dry mixing process.
Wood particles, spray dried melamine formaldehyde resin, wear resistant particles and pigments are mixed and finely dispersed in a dry form in a mixer. As the powder mixes are produced in the dry form, the distribution of the binder is limited to be on the outside of the wood particles and the binder is not distributed over the surface of the wear resistant particles and pigments. Furthermore, distribution of additives in the mix is limited in the dry mixing process. Even if added, the desired effect of the additives may not be obtained due to the non-uniform distribution of the additives.
The deep impregnation of the wood particles as well as wetting of the pigments and the wear resistant particles used for improved wear resistance, are done in the pressing process at high temperature and high pressure. The steps during the pressing should include complete melting of the binder, distribution of the binder in the matrix comprising wood particles, pigment and wear resistant particles, and impregnation and wetting of solid surfaces. These steps should be fulfilled before reaching the gel point of the binder system and after that the final curing of the binder occurs.
A failure in these conditions will cause a surface with limited staining resistance and/or a hazy surface with high opacity.
Incomplete impregnation of the binder into open parts such as cavities and channels in the wood particles causes higher opacity as well as risk for bad staining resistance. In practice, it has been proven that it is difficult to make a powder overlay based on bleached particles, binder and wear resistant particles with low opacity.
Incomplete wetting of the wear resistant particle surfaces will cause bad adhesion between the resin matrix and the wear resistant particles, which gives micro pores in the interface. These micro pores create an open surface, in which capillarity attracts stain, which is hard to remove, thereby being hard to clean.
Although the above description relates to floorings, the above description of techniques and problems thereof is applicable also for other applications, such as laminates for other purposes, for example wall panels, ceiling panels, furniture etc.
The above description of various known aspects is the applicants' characterization of such, and is not an admission that any of the above description is considered as prior art.